1. Field of the Invention
The present invention relates to an improvement in a fuse device, particularly to a current limiting fuse device for protecting a semiconductor element for use in a large current circuit from excess current.
2. Description of the Prior Art
With improvement of semiconductor element techniques in the recent years, a semiconductor rectifier element such as a diode or a thyristor may be used instead of a conventional mercury converter or power rectifier as an AC-DC or DC-AC converter for direct current transmission. As well known, however, a semiconductor element has a lower tolerance for excess current and a greater thermal fragility as compared with the mercury converter. Therefore, previously a semiconductor rectifier element having a current capacity which is far larger than the rated current, i.e., sufficiently resistant against the large current generated in case of a short-circuit accident in the direct current transmission system, has been used so as to avoid any trouble.
In this case, however, the cost of equipment becomes unavoidably expensive, therefore it is proposed that the known current limiting fuse recently used in alternating current systems be inserted in series with the semiconductor rectifier element, the fuse element being fused before a large current such as a short-circuited current reaches the current value which causes thermal breakdown of the semiconductor rectifier element so as to protect the element. However, the current limiting fuse widely used hereinbefore, a fuse device filled with quartz powder around the periphery thereof having a flat-sheet silver fuse element with a notched or narrow width portion in a pressure container has a continuous maximum current of about 1000A at the highest, but its cut-off time is very long such as more than 10-20 milliseconds. In the semiconductor converter at least 1000A is required as the continuous maximum current and a cut-off time of less than several milliseconds is required. As a result, such a fuse is incapable of protecting the semiconductor converter.
It is possible to meet the cut-off time requirement by making the cross-section of the fuse element small so as to make the fusing heat amount small, but the heat generation in the fuse element becomes large so as to lower of the continuous maximum current. Accordingly, a new technique should be used, otherwise no current limiting device for protecting a semiconductor AC-DC converter with a construction of the conventional current limiting fuse device can be obtained.
This invention improves the conventional fuse element into a hollow shape and at the same time develops a current limiting fuse device by forming a perforated cylindrical fuse device by sealing the fuse element with an arc suppression agent in one cylindrical body and immersing the perforated fuse device in a cooling and insulating oil.